Art. 39. Stone Crushing And Cost Of Aggregate

Description

Art. 39. Stone Crushing And Cost Of Aggregate

304. Breaking Stone By Hand

When but a small quantity of concrete is to be made, and broken stone cannot be purchased in the vicinity, the stone for concrete may be broken by hand. This is an extremely tedious process, however, and is generally avoided, since broken stone prepared in this way will Cost from two dollars and a half to four dollars per cubic yard. In the reconstruction of the breakwater at Buffalo, the Cost of breaking stone by hand was two dollars and eighty-six cents per cubic yard, and loading on boat Cost thirty-nine cents, making total Cost about three dollars and twenty-five cents per cubic yard.1

305. Stone Crushers

The most common forms of rock crushers are the gyratory and the movable jaw types. The jaw breaker, or Blake crusher, consists of one fixed plate or jaw and one movable one. The latter is hinged at the upper end and the lower end is moved backward and forward through a short space by means of a toggle joint or other mechanism. The jaws are several inches apart at the upper end, depending on the size of the machine, and converge toward the bottom. The distance between the jaws at the bottom regulates the size of fragments delivered, and this distance may be adjusted at will.

The Gates crusher is of the gyratory type and consists of a corrugated cone of chilled iron, called the breaking head, within a larger inverted cone, or shell, which is lined with chilled iron pieces. The vertical shaft bearing the breaking head is pivoted at the upper end while the lower end travels in a small circle ; an eccentric motion is then imparted to the head, so that it approaches successively each element of the shell. The size of opening can be regulated by raising or lowering the breaking head.

1 The stone crushing and sand and gravel washing plant used in the construction of the Canal at the Cascades of the Columbia, Ore., is described and illustrated in Report of Chief of Engineers, 1891, p. 3332.

Stone crushers are made of various sizes having capacities up to one hundred tons per hour. The Cost of running a stone crusher is not great, the principal expense being incurred in breaking the stone into pieces of proper size to feed the crusher, the delivery of the stone to the crusher, and taking it away when broken.

Crushing plants are usually provided with revolving screens into which the broken stone is delivered from the crusher. These screens are usually made of perforated steel plate, the holes being such as to separate the material into the sizes desired.

Where large amounts of concrete are required, and the stone is to be crushed on the work, the arrangement of the crusher plant should receive careful study to facilitate the transportation of the rock to and from the crusher. The broken stone should be discharged from the crusher into bins, from which the carts or cars may be filled by gravity, or from which the material may be led directly to the mixer through a chute or other form of conveyor. In quarries preparing aggregate for sale, and on important works, very complete stone crushing plants are erected.1

306. Cost Of Aggregate

The cost of aggregate varies greatly according to the proximity of the stone to the crusher, the character of the stone, and the amount required. In exceptional cases gravel suitable for use in concrete is so near at hand that it may be delivered on the mixing platform for from twenty-five to forty cents per cubic yard. When it must be brought from a distance, the Cost is correspondingly increased. Where a considerable quantity of stone is to be broken, the Cost of crushing, aside from transportation of the materials to and from the site of the work, would usually be from thirty to forty cents per cubic yard.

In one case where the stone was delivered to the crusher in carts after having been sorted from spoil banks containing much poor stone that had to be handled over, the Cost per cubic yard of crushed stone was approximately as follows for about six thousand cubic yards crushed in one season:

Labor, including sorting and delivering to crusher, per cubic yard of crushed stone............... $.67

Rent of power plant...................04

Fuel..........................05

Tools, supplies, breakages, etc...............12

Interest and depreciation of plant.............12

Total Cost per cubic yard..........$1.00

307. The following data concerning the Cost of breaking a large amount of stone for road material are given by Messrs. Spielman and Brush.1 "The stone was broken by a ten-inch Blake stone crusher at the rate of about twenty cubic yards in ten hours. The size of the stones as they came from the crusher was: fifty per cent., two inches size; twenty-five per cent., one and one-half to one inch size; twenty-five per cent., screenings and pea dust. The Cost of the crusher, engine, boiler, etc., set up complete, was about twenty-five hundred dollars. The cost of working per day independent of the original cost of the machinery and interest thereon, and also independent of any royalty on the stone, was found by the contractor to be as follows: —

308. The Cost of breaking trap on the Palisades is given as follows:2 "Two crushers deliver thirty-five cubic yards of twoinch stone per day. when working well, the stone being sledged to go into the jaws readily; fifteen per cent, of the time is lost by breakdowns: —

Breakages.................. 5.00 $17.00 or about fifty-seven cents per cubic yard.

"On Snake Island, three crushers were arranged in a row, and the broken stone was carried by an endless belt to the revolving screen, whence it fell into the bins, so that no screen-ers were employed. The engine had one cylinder eight inches by twenty-four inches, and was running with eighty pounds of steam. The product was said to be one hundred eighty cubic yards per day when there was no breakdown." The Cost was as follows:1 —

1 engineman and fireman........... $2.50

3 laborers feeding, at $1.25.......... 3.75

2 1/2 tons coal, at $3.50............. 8.75

Oil, etc.................... 2.00

Breakages.................. 15.00 $32.00

"Allowing for the fifteen per cent, lost by breakdowns, the Cost would be about twenty-one cents per cubic yard".

At another place on the Hudson, two crushers, set face to face, nine-inch by fifteen-inch jaws, could deliver at the rate of one hundred twenty cubic yards per day when no trouble occurred, but one hundred cubic yards was a fair average.

In the construction of the defenses at Portland, Me.,1 a No. 5 Champion Crusher was used, driven by a thirty horse-power portable engine. Granite was purchased at one dollar per ton on the wharf. Hauling to crusher Cost thirteen cents per ton. Cost of crushing, twenty cents per cubic yard of crushed stone, making total Cost of crushed stone in bin at crusher one dollar eighty-three cents per cu. yd.